By Lydia Wong

This post is an excerpt from updates sent to those following Lydia’s research project on the effects of urban warming on cavity-nesting insects. If you’d like to receive these updates, please contact Lydia at lwong014@uottawa.ca.

Hello there, cavity-watchers!

I hope everyone is doing well and settling nicely into winter. I finished collecting hotels and temperature sensors from all 34 sites back in November, then spent a couple of days in the lab, working through the contenct of the hotels that had hosted bees and wasps this summer.

Most of the nests this year came from leafcutter bees. Grass-carrying wasp nests were the second most numerous, followed by a spatter of other cavity-dwellers.

 

There are so many interesting quirks to all of these occupant types, and I think each deserves a letter of its own… so I’ll start with the leafcutters (Megachile species).

Using a “cocoon-scooper” (nothing fancy… just a plastic knife carved to fit the width of the cavities), I scooped the entire series of cells out of the cavities.

I then carefully separated the cells with my fingers.

I’d then peel the leaves surrounding the cocoon off with super fine forceps until I got to the cocoon (I needed to weigh the cocoons without all the leaves).

Female leafcutter bees mostly cut two leaf shapes – the more circular ones are for the “head” of the cell while the oval-shaped ones wrap around the developing baby bee. Each cell usually has multiple layers of leaves (multiple circle and multiple oval pieces).

It was neat seeing the little circles of carefully cut leaves all strewn about on the lab bench, although I felt a twinge of guilt for having undone such meticulous work of the mother bee. (Note: all the cocoons are now safely overwintering in small vials outdoors, despite the fact that I removed their leaves.)

Of all the hotel occupants, the leafcutter bee nests seemed to have been the most vulnerable to earwigs and other parasites. In some cases, I could see the damage even before opening the nests. This nest at the Fletcher Wildlife Garden in Ottawa was quite badly “earwigged” (photo below).

The leaves and nest contents (pollen and developing bees) had been chewed quite thoroughly, and I was left with copious amounts of poop.

In other cases, the nest looked deceptively intact on the outside, only to reveal a row of completely empty cells all the way through.

These nests were likely the victims of chalcid wasps — small parasitic wasps (probably Monodontomerus or Mellitobia species). These wasps are so tiny (often just a few millimetres in length), they were probably able to squeeze into the tiny cracks I left between the plastic pane and wood of the hotels.

Crack I left between the plastic and wood.

Once in the nest, the female wasps insert their long ovipositors (egg-laying organ) through the leaves of the cell and lay numerous eggs at once. The eggs develop into larvae very rapidly (much faster than the host bee), eating the baby bee in the cell alive as they go… Here are photos (not mine) of a female Monodontomerus ovipositing into a cocoon (top), and a cocoon full of parasite larvae (bottom). Photos by D.F. Veirs.

I found this video taken by George Pilkington of this all happening in real-time. A bit sad – from the bee’s perspective.

Unlike the bees they parasitize, these little wasps can emerge as adults the same year they were born (they don’t need to wait for spring to emerge as adults). To exit from the now bee-baby-less cell, the wasps chew miniscule “exit” holes when they emerge.

I lost close to 80% of the leafcutter bee cocoons to earwig and parasitic wasp damage, which I’ll admit was a little demoralizing. Not to mention, I felt sorry for the poor mother bees who had obviously spent a lot of time provisioning their nests with pollen and nectar and carefully wrapping their children in two sizes of leaves. This is definitely something I’ll have to think about for next summer (minimizing gaps/cracks in the design of the boxes and designing something to reduce earwig occupancy). At the same time, I try to remind myself to see things evenly through the perspectives of all critters – parasitic wasps have their place in this world too. (That said, whether or not 80% loss to earwig and parasitic wasps reflects “natural” conditions is another question – again, very likely a hotel design problem.) I suppose things just get a little hairy from my perspective as a student when they happen to be eating my data!


Lydia Wong is a graduate student working with Dr. Jessica Forrest at the University of Ottawa. Her research explores the impacts of warming and drying climates on wild pollinators. She spends the summers studying this in the Colorado Rockies but can also be found nosing around the concrete jungles of big cities on the lookout for city-dwelling bees and wasps. She is currently working with citizen scientists in Toronto and Ottawa to explore the effects of urban warming on cavity-nesting bees and wasps.

One response to “Cavity nesters: leafcutter bees”

  1. Lori Fernandez says:

    This is fascinating!! As a non-bee expert, I think you did a great job explaining the more complex parts of cavity nesting and being honest about the strenuous process of collecting this data! As a BSc student at uOttawa hoping to secure a co-op field season in the summer related to bees, I found this article very insightful – thank you so much! I look forward to following your work, Lydia 🙂

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